The present invention relates to regulated voltage supplies, and more particularly to regulated voltage supplies having back-up batteries that must be protected from overcharging.
Power field effect transistors (FETs) have a high OFF impedance, a low ON impedance, and very good temperature stability. These characteristics make power FETs a near ideal regulator device for power supplies. In such regulator applications, however, the presence of reverse voltage protection diodes, which are included in most power FETs to prevent electrostatic discharge damage, must be kept in mind. The reverse voltage protection diode of primary interest in regulator designs is connected between the drain and source terminals. In an N-channel FET (N-FET), this drain-to-source diode has its anode connected to the source terminal and its cathode connected to the drain terminal. Conversely in a P-channel FET (P-FET), the drain-to-source diode has its anode connected to the drain terminal and its cathode connected to the source terminal.
FIGS. 1A and 1B show a power FET application in a power supply having a back-up battery. If Q9 is replaced with a short circuit between its drain and source connections and if its gate connection is open circuited, then FIGS. 1A and 1B would be equivalent to the power supply that is the subject of co-pending, allowed U.S. patent application Ser. No. 371,564, which is hereby incorporated by reference. The power supply shown in this allowed Application is of the magnetic amplifier type, but it could just as easily be another type of regulated power supply, such as a series and/or a shunt regulated power supply. The N-channel power FET Q10 is used as a switch and as a series regulator to supply voltage and current from a back-up battery 24 if the AC line power should fail.
Under normal circumstances a battery charger (not shown) will keep the battery charged with a controlled current and voltage to a level that is higher than the normal output voltage, thereby keeping the drain-to-source diode D10 reversed biased, which prevents current flow therethrough. A serious problem can arise, however, if the output voltage of the magnetic amplifier regulated power supply exceeds the fully charged voltage of the back-battery. This situation may occur if the output voltage regulator circuit has a component failure causing the output voltage to rise, or if the back-up battery has a short circuit in one or more of its electrolytic cells causing the level between its + and - terminals to drop. In either case, the cells of the battery may be overcharged through the drain-to-source diode. An overcharged battery cell may emit large amounts of hydrogen gas, a highly explosive gas, and may boil caustic electrolyte out of its cells, a corrosive liquid that is hazardous to personnel and electronic components alike.
One known way of preventing such a reverse overcharge is to use a series blocking diode which has its forward current flow direction opposite to the forward current flow direction of protection diode D10 to prevent any overcharging current. However, such blocking diodes typically have a forward voltage drop of 0.7 to 1.0 volts, which is a substantial drop for a 5.0 volt supply. Additionally, such blocking diodes have high forward series impedances that are many times greater than the series impedance of a FET in the ON condition. The relatively high voltage drop and the relatively high series impedance of a blocking diode substantially limit the operating power levels and operating time that the back-up battery can deliver during an AC power line failure.
Thus, it is an object of the present invention to provide a regulated power supply having a back-up battery without the hazards of overcharging one or more of the back-up battery cells.
It is another object of the present invention to provide a regulated power supply having a back-up battery that has a longer available back-up time by the use of low impedance FETs as the series regulator and/or switch.